BACKGROUND: CDK4/6 regulates the G1-S phase transition by phosphorylating the retinoblastoma protein (Rb). Given their potent clinical efficacy, CDK4/6 inhibitors used in combination with hormone receptor (HR) blockade (with an aromatase inhibitor or fulvestrant) are emerging as the standard of care for patients with metastatic HR-positive breast cancers. The CDK4/6 inhibitors palbociclib and ribociclib are FDA-approved for use in HR-positive breast cancer patients, and abemaciclib is currently in phase III trials. We observed that approximately 74% (25/34) of breast cancer cell lines had high phosphorylated Rb (phospho-Rb) expression levels and that triple-negative breast cancer (TNBC) cell lines often expressed phospho-Rb, suggesting that targeting phospho-Rb via CDK4/6 inhibition may be effective against TNBC. The histone deacetylase (HDAC) inhibitors increase p21Cip1 levels, promoting proteasomal degradation of cyclin B1 and resulting in G2/M arrest. Entinostat is an oral, class 1, selective HDAC inhibitor currently in phase III testing in HR-positive breast cancer. Preclinical and clinical data demonstrate that entinostat, in combination with HR blockade, has anticancer activity. Our group recently reported that entinostat combined with other anticancer drugs induced apoptosis via induction of proapoptotic proteins such as Noxa and Bim in breast cancer cell lines. Based on these findings, we hypothesized that entinostat-induced apoptosis and palbociclib-induced cell cycle arrest synergize to produce enhanced antitumor effects in estrogen receptor (ER)-positive breast cancer and TNBC cell lines with high phospho-Rb expression levels.

METHODS: We assessed the combination antitumor effects and their mechanisms via CellTiter Blue and sulforhodamine B assays, flow cytometry, apoptosis (caspase 3/7) assays, anchorage-independent growth assays, Western blotting, reverse phase protein array (RPPA), and mammary fat pad xenograft mouse models.

RESULTS: RPPA data showed that ER-positive and TNBC cell lines more often expressed phospho-Rb than did other breast cancer cell subtypes (7/10 and 8/17 cell lines, respectively). We found that the combination of entinostat and palbociclib synergistically inhibited tumor cell proliferation (combinational index less than 1.0), reduced in vitro colony formation (P < 0.05), inhibited in vivo tumor growth in ER-positive MCF-7 breast cancer cells (P < 0.05), and inhibited tumor growth in TNBC xenograft mouse models (MDA-MB-231) more effectively than did either drug alone.

CONCLUSION: Taken together, our data provide evidence that combining entinostat with palbociclib enhances the antitumor effects of these drugs. Along with our continued effort to determine predictive biomarkers, our findings justify conducting a clinical trial of combination treatment with entinostat and palbociclib in patients with ER-positive breast cancer or TNBC.

Citation Format: Lee J, Lim B, Pearson T, Tripathy D, Ordentlich P, Ueno NT. The synergistic antitumor activity of entinostat (MS-275) in combination with palbociclib (PD 0332991) in estrogen receptor-positive and triple-negative breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-21-15.